The present invention relates to wireless communications, and more particularly to systems and methods for detecting discontinuous transmission in wireless communications.
Discontinuous transmission (DTX) is widely used in terrestrial and satellite cellular radiotelephone systems, including systems based on Time Division Multiple Access (TDMA), such as Global System for Mobile (GSM) communications and TIA/EIA-136, or Code Division Multiple Access (CDMA), such as IS-95. In particular, in conventional cellular radiotelephone communications systems, a wireless terminal may transmit continuously during a call. Normal conversation, however, may generally include pauses between periods of speech, such as when a user listens to the other party. When a wireless terminal user is not speaking, transmission of the radio signal may not be needed from an information point of view. With discontinuous transmission, pauses in normal speech are detected in order to suspend radio transmission for the duration of the pause. Discontinuous transmission can reduce air traffic, reduce interference between users, and/or extend battery life in wireless terminals. It will be understood that discontinuous transmission also may be used by a base station of a terrestrial network or a space-based component of a satellite network to wirelessly transmit to wireless terminals.
When using discontinuous transmission, a wireless transmitter may include a speech activity detector that detects whether or not a speech signal is present. During the periods during which no speech activity is detected, the transmitter may transmit nothing (as in a GSM system), or may transmit truncated bursts containing only radio control information (for example, 34 symbols versus 162 symbols, as in a TIA/EIA-136 system). The transmission of at least some truncated bursts may be desirable, to maintain a wireless connection between the wireless terminal and the base station serving it, and to transmit control information. In either case, the state in which truncated bursts are transmitted also is known as a DTX-Low or DTX-0 state, and the state in which normal, full-length bursts are transmitted also is known as a DTX-High or DTX-1 state.
Accordingly, it may be desirable for the wireless receiver to be able to distinguish between the DTX-High state and the DTX-Low state, in order to detect whether a normal burst or a truncated burst is present in a received information signal.
It is known to detect whether a normal burst or a truncated burst is present in a received information signal by using one or more features that are calculated during the processing (demodulating and/or decoding) of each received burst. For example, U.S. Pat. No. 6,097,772 to Johnson et al., entitled System and Method for Detecting Speech Transmissions in the Presence of Control Signaling, assigned to the assignee of the present invention, the disclosure of which is hereby incorporated herein by reference in its entirety as if set forth fully herein, describes the use of an estimated bit error rate to reduce the probability that control signals and other non-speech transmission segments are interpreted as speech and played. Also, U.S. Pat. No. 6,092,230 to Wood et al., entitled Method and Apparatus for Detecting Bad Frames of Information in a Communication System, the disclosure of which is hereby incorporated herein by reference in its entirety as if set forth fully herein, describes the use of a Viterbi decoder metric to detect bad frames of information. Finally, U.S. patent application Ser. No. 10/002,722, filed Nov. 15, 2001, corresponding to U.S. Publication No. U.S. 2003/0095507 A1, published May 22, 2003, entitled DTX Detection Method With High Success Probability, to the present inventor, et al., and assigned to the assignee of the present invention, the disclosure of which is hereby incorporated herein by reference in its entirety as if set forth fully herein, describes the use of the Euclidian distance between a soft bit representation of a reference field, and the received frame data bits corresponding to the position of the reference field in a truncated burst, to provide discontinuous transmission detection. However, the use of at least one feature that is calculated during the processing of each received burst may not provide adequate detection probability in high noise environments.